Adjust gradient backgrounds to match designs

Two primary changes:
- Reset the tonal elevation inside NiaBackground to 0.dp, to prevent additive tonal elevation
- Change the gradient background to match Figma: 2 overlapping gradients with a slight angle

Change-Id: I183f67f5ca16acbc5125a52131bbd14c6406ad72

core-ui/src/main/java/com/google/samples/apps/nowinandroid/core/ui/component/Background.kt

@@ -17,13 +17,18 @@
 package com.google.samples.apps.nowinandroid.core.ui.component
 
 import android.content.res.Configuration
-import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.size
+import androidx.compose.material3.LocalAbsoluteTonalElevation
 import androidx.compose.material3.Surface
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.CompositionLocalProvider
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.rememberUpdatedState
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.drawWithCache
+import androidx.compose.ui.geometry.Offset
 import androidx.compose.ui.graphics.Brush
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.tooling.preview.Preview
@@ -31,10 +36,11 @@ import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 import com.google.samples.apps.nowinandroid.core.ui.theme.LocalBackgroundTheme
 import com.google.samples.apps.nowinandroid.core.ui.theme.NiaTheme
+import kotlin.math.tan
 
 /**
  * The main background for the app.
- * Uses [LocalBackgroundTheme] to set the color and tonal elevation of a [Surface].
+ * Uses [LocalBackgroundTheme] to set the color and tonal elevation of a [Box].
  *
  * @param modifier Modifier to be applied to the background.
  * @param content The background content.
@@ -50,13 +56,16 @@ fun NiaBackground(
         color = if (color == Color.Unspecified) Color.Transparent else color,
         tonalElevation = if (tonalElevation == Dp.Unspecified) 0.dp else tonalElevation,
         modifier = modifier.fillMaxSize(),
-        content = content
-    )
+    ) {
+        CompositionLocalProvider(LocalAbsoluteTonalElevation provides 0.dp) {
+            content()
+        }
+    }
 }
 
 /**
- * A gradient background for select screens, to be overlaid on top of [NiaBackground].
- * Uses [LocalBackgroundTheme] to set the gradient colors of a [Box].
+ * A gradient background for select screens. Uses [LocalBackgroundTheme] to set the gradient colors
+ * of a [Box].
  *
  * @param modifier Modifier to be applied to the background.
  * @param topColor The top gradient color to be rendered.
@@ -70,21 +79,56 @@ fun NiaGradientBackground(
     bottomColor: Color = LocalBackgroundTheme.current.secondaryGradientColor,
     content: @Composable () -> Unit
 ) {
-    val gradientModifier = Modifier.background(
-        Brush.verticalGradient(
-            listOf(
-                if (topColor == Color.Unspecified) Color.Transparent else topColor,
-                Color.Transparent,
-                if (bottomColor == Color.Unspecified) Color.Transparent else bottomColor
-            )
-        )
-    )
-    Box(
-        modifier = modifier
-            .fillMaxSize()
-            .then(gradientModifier)
-    ) {
-        content()
+    val currentTopColor by rememberUpdatedState(topColor)
+    val currentBottomColor by rememberUpdatedState(bottomColor)
+    NiaBackground(modifier) {
+        Box(
+            Modifier
+                .fillMaxSize()
+                .drawWithCache {
+                    // Compute the start and end coordinates such that the gradients are angled 11.06
+                    // degrees off the vertical axis
+                    val offset = size.height * tan(
+                        Math
+                            .toRadians(11.06)
+                            .toFloat()
+                    )
+
+                    val start = Offset(size.width / 2 + offset / 2, 0f)
+                    val end = Offset(size.width / 2 - offset / 2, size.height)
+
+                    // Create the top gradient that fades out after the halfway point vertically
+                    val topGradient = Brush.linearGradient(
+                        0f to if (currentTopColor == Color.Unspecified) {
+                            Color.Transparent
+                        } else {
+                            currentTopColor
+                        },
+                        0.724f to Color.Transparent,
+                        start = start,
+                        end = end,
+                    )
+                    // Create the bottom gradient that fades in before the halfway point vertically
+                    val bottomGradient = Brush.linearGradient(
+                        0.2552f to Color.Transparent,
+                        1f to if (currentBottomColor == Color.Unspecified) {
+                            Color.Transparent
+                        } else {
+                            currentBottomColor
+                        },
+                        start = start,
+                        end = end,
+                    )
+
+                    onDrawBehind {
+                        // There is overlap here, so order is important
+                        drawRect(topGradient)
+                        drawRect(bottomGradient)
+                    }
+                }
+        ) {
+            content()
+        }
     }
 }